Design Tips For Incorporating GSM Modules into an Embedded System

As loving and rewarding as a family is, sometimes they can be a pain. My aunt, for example, refuses to communicate with my grandfather. They live less than thirty minutes apart; however, refuse to see each other. This makes coordinating visiting my family unnecessarily difficult as, often, I need to make separate day plans in order to see each so-to-speak faction. But what’s most stressful, is that often it leaves the middleman communication role to me: if there is something going on with either my aunt or my grandfather, then I am the one who has to inform either of them. This process takes a lot of mental energy and requires a lot of keeping track.

Serving as a middle link for communication can be exhausting. This is especially true for mobile devices such as tablets and phones when you integrate a Global System for Mobile Communications (GSM) module with an embedded system. While I don’t often empathize with machinery and modules, the difficulties with maintaining power integrity and machinic exhaustion were feelings that I often felt when trying to plan out alternating holiday parties.

Mobile Devices and Global Systems for Mobile Communications

A GSM module is used to set up communications between an embedded system and the GSM cellular network. The GSM operates at different frequencies worldwide. Frequencies of 900Mhz and 1800 MHz are commonly used in Europe, Asia, Oceania, and the Middle East, while the United States uses 950 Mhz and 1850 Mhz.

A GSM module enables the embedded system to send and receive text messages, send data on the General Packet Radio Service (GPRS) network, and make or receive voice calls. As with a regular mobile phone, the GSM module requires an activated SIM card to operate.

The age-old GSM technology is also widely used in other kinds of applications, including vending machines and energy systems. GSM technology enables an embedded system to transfer operational data to a central server without the need for human intervention. A GSM module could be used for anything as convenient as vending machines capable of alerting suppliers when ingredients need to be restocked, or as particularly helpful as energy efficiency systems that monitor electrical parameters and enable building management teams to control the system remotely.

Some vending machines use GSM modules to keep track of inventory.

How a Microcontroller Integrates with a GSM Module

GSM modules are commonly available in a ready-to-mount PCB format, together with a SIM card socket and an antenna jack. Modules are also available in an integrated circuit (IC) package, but that requires you to design the complete circuitry for the GSM IC.

The SIM900A is a popular GSM module that I’ve used in my designs. The SIM900A IC operates at a range of 3.4V to 4.4V. However, it has a peak current that can go beyond 2A, and this can affect the way you design your PCB. The SIM900A includes some critical communication circuitry for the GSM module— with the microcontroller using the Universal Asynchronous Receiver-Transmitter(UART) and a connection to the SIM card.

The microcontroller uses the standard AT&T protocol to communicate with the GSM module. Operations like sending and receiving text messages are completed by sending the correct command AT&T sequence to the GSM module. This shouldn’t be an issue for an experienced firmware developer unless the hardware design is at fault to start with.

Best Practices When Designing a PCB with a GSM Module

In my first ever prototype, I spent hours trying to discover the reason why the microcontroller reset itself each time it tried to send text messages. After ruling out runaway codes, bad pointers, and stack overflow, I finally realized that the voltage regulator was insufficient to withstand the current drained by the GSM module during data transmission.

One of the common issues that plague GSM module design is the limited power supply capacity. It isn’t difficult to ensure that the GSM module receives the correct voltage level, but the trick is in ensuring that the power is adequate when transmitting data. A typical GSM module may draw more than 2A when transmitting.

You need to ensure that the voltage regulator that’s supplying the GSM module is able to handle the sudden spike in current. Not only that, the power supply copper connection has to be wide enough and thick enough to handle the high current. Otherwise, you’ll risk damaging the copper track itself. It is also important to use proper heat dissipation techniques for the power management circuit, as it can produce a great amount of heat.

Electromagnetic interference (EMI) can also be a problem that affects the stability of your embedded system. An antenna is usually connected to the GSM module to boost the radio wave signal strength. The whole system needs to undergo stringent testing to ensure the microcontroller is not affected by the EMI, particularly during transmission and reception. Common best practices, such as ensuring adequate clearance between GSM and other onboard modules, helps in reducing EMI problems.

Ensure that the power management circuitry can handle current draw from the GSM module.

The GSM module introduces new challenges and uncertainties into a design. While you can’t design around your family, you can design around power integrity. This is where using tools like Altium Designer’sPDN Analyzer will ensure that the current density in the power connections is at the appropriate level.